1. Field of the Invention
The present invention relates to an air compressor for generating a compressed air for use with an air tool such as a pneumatic nailing machine, and more particularly to an air compressor having long continuous working time up to reaching a working critical pressure, and a pressure switch mechanism for use therewith.
2. Description of the Related Art
Generally, the air compressor for use with the air tool compresses the air sucked via a suction valve by rotationally driving a crank shaft of a compressor main body with a motor, and reciprocating a piston within a cylinder by rotations of the crank shaft. And a compressed air produced in the compressor main body is discharged via an exhaust valve through a pipe into an air tank, and reserved within the tank. The air tool makes the nailing operation or the like, employing the compressed air reserved within this tank.
Some of the air compressors for the air tool constituted in the above manner may be sometimes used as an installed type, but mostly used as a portable type, and brought into the building site for use. Accordingly, the discharge amount, namely, compressed air production capability is relatively small in most cases. Also, it is required that the air compressor is as small as possible and has excellent portability.
For the reason of safety, the compressor has a function of stopping the motor, if the pressure within tank reaches a specified value, and restarting the motor if the pressure within tank falls below the specified value by the use of the air tool. A pressure switch implements this function. That is, the pressure switch detects the air pressure within tank with a pressure sensor, and turns on or off the motor in accordance with a signal from the sensor.
FIG. 6 shows the operation of the conventional pressure switch. The longitudinal axis represents the air pressure within tank (kg/cm2), and the transverse axis represents the time (min). Reference sign PH denotes a pressure switch off point at the time of pressure buildup, PB denotes a pressure switch on point at the time of pressure fall, and PL denotes a working critical pressure.
In the prior art, PB is set at such a value as PB=(0.8 to 0.85)*PH. The reason is that since the pressure within tank gradually decreases due to a fall in temperature within tank and air leakage after the motor is stopped at PH point, if there is a small difference between PH and PB, the motor is repeatedly started and stopped at high frequency without use of the air tool, and oscillated. Therefore, PB is set at such value to prevent occurrence of this phenomenon.
In FIG. 6, if the motor is started in a state where there is no air consumption and the pressure within tank is zero, the pressure within tank increases, and the motor is stopped at point a of reaching PH. If the air tool with smaller air consumption is continuously employed immediately after the motor is stopped, the pressure within tank decreases relatively gradually, and the motor is restarted at point b of reaching PB. And the pressure within tank increases again, and at point c of reaching PH, the motor is stopped. If the air tool with a large amount of air consumption is continuously employed immediately after the motor is stopped at point c, the pressure within tank decreases rapidly, and at point d of reaching PB, the motor is restarted. However, because the air production amount does not follow the air consumption amount, the pressure within tank continues to fall, and finally at point e, the working critical pressure PL is reached, making it impossible to continue the subsequent operation. In this case, it is required that the operation is interrupted to wait until the pressure within tank increases, and then the next operation is performed.
Accordingly, there is a problem with the prior art that when the air tool with a large amount of air consumption is employed, the continuous serviceable time is shorter.